Neutron source



1963 J. L. RICHMOND ET AL 3,

NEUTRON SOURCE Filed Oct. 17, 1960 4 Fig.1

IN VE N TORS John L. Richmond Charles E. Wells A f Iarney v smarts Patented Jan. 15, 1963 3,073,768 NEUTRGN SOURCE John L. Richmond, lviiamishurg, and (ibarles E. Wells,

Farmersvilie, Ghio, assignors to the United States of America as represented by the United States Atomic Energy Qomrnission Filed Get. 17, 1960, der. No. 63,231 6 Claims. (Ci. 204-1541} The present invention pertains to neutron sources, which are now widely used in industry for many purposes, and more particularly to .a new and improved method of making plutonium-beryllium neutronsources and to neutron sources produced thereby.

Plutonium-beryllium neutron sources have previously been produced but they and their methods of manufacture have not been entirely satisfactory. For example, in order to obviate or minimize the serious objection of beryllium contacting and reacting, alloying or combining with an outer metal container (generally tantalum) during the manufacturing process, it has been customary to employ a separate beryllium oxide (beryllia) receptacle to hold the charges of plutonium and beryllium that are proportioned to give the reaction product PuBe thus contact is prevented between the charge and an outer metal container during the requisite heating and effecting of the reaction producing neutron-emitting PuBe The particular neutron yield required of a source is generally specified in advance and in order to provide substantially that particular yield and no other, the quancities of the elements plutonium and beryllium must be accurately predetermined in order that when correct weights thereof are put together and heated to their react ing temperature (an exothermic reaction) the final PuBe composition will be that which gives approximately the required neutron emissions per second. If the beryllium portion of the charge contacts the outer metal container at least some of it reacts, alloys with, or permeates the container and the appropriate proportion of plutonium and beryllium is substantially upset, so that a resulting neutron source does not provide the desired neutron emissions per unit of time. The separate 'beryllia receptacle has previously been employed to overcome such difficulty by providing a substantially inert container intermediate the reacting products and the outer metal container.

The previous process is relatively slow and the beryllia receptacles are expensive. Subsequent steps of breaking the P718313 compound away from the beryllia receptacles, putting the compound in a metal housing, and then silver soldering and nickel coating the housing exterior subjects personnel to possible radiation hazard. If it is attempted to expedite manufacture and also reduce the radiation hazard possibility by enclosing the entire beryllia receptacle (with the produced PuBe therein) into a metal housing, other drawbacks arise, e.g., the beryllia receptacle walls tend to defeat the very purpose of the neutron emitting source in that the beryllia walls reflect neutrons back into the interior of the receptacle, rather than let them escape. Also, it is found that such a neutron source does not provide optimum symmetry of neutron emission readings about its exterior.

The present invention aims to provide a new and improved neutron source and method of making it which overcomes or minimizes the above and other difficulties or disadvantages.

An object of the present invention is to provide a new and improved method of making neutron sources which is faster and safer than previous methods.

Another object of the invention is to provide an im proved method of making neutron sources which does not require employment of a separate beryllia receptacle.

Still another object of the invention is to provide a method of making neutron sources of improved yield.

A further object of the invention is to provide a neutron source which gives improved symmetry of neutron emission.

A still further object of the invention is to provide an improved neutron source which is less'costly to manufacture.

Other and further objects of the invention will be obvious upon an understanding of the illustrative embodiment about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the .art upon employment of the invention in practice.

Preferred embodiments of the invention have been chosen for purposes of illustration and description. The preferred embodiments are not intended to be exhaustive nor to limit the invention to the precise forms disclosed. They are chosenand described in order to best explain the principles of the invention and their application in practical use to thereby enable others skilled in the art to best utilize the invention in various embodiments and modifications as are best adapted to the particular use contemplated.

In the accompanying drawings:

PEG. 1 is a sectional view showing the relationship of certain components of one form of the present improved neutron source at a stage in the manufacture thereof.

FIG. 2 is a view somewhat similar to FIG. 1 but showing a neutron source container of another configuration.

As shown in FIG. 1, the components comprise an inner beryllium crucible or cylinder 1 adapted to hold an appropriate quantity of plutonium pieces 2 and to react therewith to produce neutron-emitting PuBe of desired yield. While a cover may extend over and close the top or opening of the beryllium crucible 1, such is not generally desirable as its use adds to fabrication and handling costs. Outwardly disposed surfaces of the beryllium crucible and cover carry or are enveloped by an inert isolating layer or stratum 5 of beryylium oxide and/or beryllium nitride, the formation of which will be later described. The crucible is in turn enclosed by an outer container 8, preferably of tantalum but feasibly molybdenum or tungsten, and its closely machined and tapered tantalum plug 9 fits tightly against and into the container tapered mouth. The plug 9 and container 3 are preferably joined together, after evacuation of gases from the crucible and tantalum container, by heliarc welding around their intermediate joint it) and during this welding the container is preferably rotated at some optimum speed to minimize the possibility of uneven heating and distortion of the container or plug. In this general condition the combination is heated, in an induction furnace and when a temperature of about ll50 C. is reached the plutonium reacts with the beryllium, an exothermic reaction which results in formation 0f PuBe The exterior tantalum container is usually subsequently fitted with a snug stainless steel jacket (not shown) that carries appropriate identifying data.

The preferred isolating layer 5 on outer surfaces of the beryllium crucible keeps the beryllium from objectionally reacting with the metal forming the container 3 and plug 9.- As one example, the insert layer 5 may be formed by first heating, for about 15 minutes, the cylinder *1 to about l0001100 C. in the presence of air at about one pound per square inch 0 pressure and thereafter admitting to the heating system and the cylinder nitrogen gas at approximately atmospheris pressure. The isolating beryllium oxide-nitride layer formed is somewhat thin but affords good protection during the previously referred to reaction.

While it is feasible to heat a beryllium cylinder in air, for about 15 minutes and at a temperature of 800 C. to 1000 C, to form a protective oxide layer, such is not preferred as the layer does not tend to hold in position quite as Well as the combination beryllium oxide-nitride layer or the beryllium-nitride layer alone. The beryllium cylinder thus formed should, of course, be of appropriate weight or quantity for reacting with an approprite weight or quantity of plutonium to form the compound PuBe and this weight may be achieved by grinding or drilling out the inside of the beryllium cylinder to bring it to the proper weight, as well as to insure that inner surfaces of the crucible are solely of beryllium and free of any layer thatmight interfere with reaction between the beryllium of the crucible and plutonium therein, to form PuBe As the layer amounts to less than 0.1% of the beryllium weight it is of no great concern.

In FIG. 2, the various components are shown adapted to provide a neutron source of spherical shape, instead of cylindrical as in FIG. 1. The spherical shape tends to give neutron emission characteristics which are more nearly symmetrical entirely about the spheres exterior surface. Reference numerals corresponding to those of FIG. 1 have been applied to FIG. 2 components which are generally similar to those shown in FIG. 1.

While various appropriate quantities of plutonium and beryllium may be combined to provide neutron sources of desired yields the following are instances:

1) A beryllium crucible of the type shown in FIG. 1, and referred to in the first of the aforementioned exam ples, of weight 1.97 grams was charged with 4 grams of plutonium and brought to the reacting temperature. The resulting source emitted 3.72 l0 neutrons per second.

(2) A beryllium crucible of the type shown in FIG. 1, and treated as referred to in the second of the aforementioned examples, of weight 4.88 grams was charged with 9.05 grams of plutonium and brought to the reacting temperature. This resulting source emitted 8.74 10 neutrons per second.

(3) A beryllium crucible of the type shown in FIG. 2 and treated as referred to in the second of the aforementioned examples, of weight 3.49 grams was charged with 7.02 grams of plutonium and brought to the reacting temperature. The resulting source gave 6.35X 10 neutrons per second. I

What happens to the inert isolating or protective layer of beryllium oxide and/or beryllium nitride during the exothermic reaction between the beryllium and plutonium metals, which is completed in only a few seconds, is not entirely clear. However, the inert layer, which is of negligible quantity insofar as any detrimental etfects are concerned, seems to exist long enough to prevent the crucible and contents from objectionally reacting or combining with the outer container (preferably tantalum) walls. In any event, neutron sources produced in accordance with the described method are highly satisfactory, may be manufactured in less time than by the previous method, are less costly to manufacture, and afford a safer process by subjecting personnel to less potential radiation hazard.

As various changes may be made in the form, construction and arrangement of the parts herein without departing from the spirit and scope of the invention and without sacrificing any of its advantages, it is to be understood that all matter herein is to be interpreted as illustrative and not in a limiting sense.

We claim:

'1. The method of making a neutron source which comprises the steps of providing at exposed surfaces of a beryllium crucible an isolating layer of at least one material selected from the group consisting of beryllium oxide and beryllium nitride by subjecting said surfaces to at least one substance selected from the'group consisting of air and nitrogen, placing in said crucible a quantity of plutonium, enclosing said crucible within a metal container and at least partially evacuating said container, sealing said container, and thereafter heating said container and contents to a temperature such as to react said beryllium and plutonium and form PuBe 2. The method as claimed in claim 1, wherein said isolating layer is formed by first heating the beryllium crucible in air, subsequently heating the beryllium crucible in an atmosphere of nitrogen, and thereafter removing at least aportion of the resulting isolating layer to form a hollow interior devoid of said layer.

3. The method as claimed in claim 2, wherein said first heating is conducted at a pressure of about 1 pound per square inch for a period of about 15 minutes and said subsequent heating is conducted at a pressure of about 15 pounds per square inch for a period of about 15 minutes.

4. The method of making a neutron source which comprises the steps of subjecting exposed outer surfaces of a beryllium crucible to air and nitrogen for forming an isolating layer consisting essentially of beryllium oxides and beryllium nitrides, placing in said crucible a quantity of plutonium, enclosing said crucible within a tantalum container and at least partially evacuating said container, sealing said container, and thereafter heating said container and contents to a temperature such as to react said beryllium and plutonium and form PuBe 5. The combination for producing a neutron source comprising a beryllium inner container enclosing a quantity of plutonium, an outer metal container enclosing said inner container, and an inert layer of material comprising beryllium nitride integral with and on the outer surface of said inner container for preventing reaction between said beryllium and metal when heated to the temperature for effecting exothermic reaction between beryllium and plutonium.

6. The combination for producing a neutron source comprising a beryllium inner container enclosing a quantity of plutonium, an outer metal container enclosing said inner container, and an inert layer of material comprising a combination of beryllium oxide and beryllium nitride integral with and on the outer surface of said inner container for preventing reaction between said beryllium and metal when heated to the temperature for effecting exothermic reaction between beryllium and plutonium.

References Cited in the file of this patent UNITED STATES PATENTS 2,863,816 Stacy Dec. 9, 1958 2,864,758 Shackelford Dec. 16, 1958 2,870,339 Birden Jan. 20, 1959 2,956,000 Kendall et a1. Oct. 11, 1960 OTHER REFERENCES Tate et al.: Plutonium-Beryllium Neutron Sources, Their Fabrication and Their Yield, 2nd United Nations International Conference on the Peaceful Uses of Atomic Energy, Geneva, 1958, P/700, pp. 430 and 431 relied upon.

Rimbach et al.: Beryllium, Reinhold Publishing Corp., 1932, p. 94 relied upon. 

1. THE METHOD OF MAKING A NEUTRON SOURCE WHICH COMPRISES THE STEPS OF PROVIDING AT EXPOSED SURFACES OF A BERYLLIUM CRUCIBLE AN ISOLATING LAYER AT LEAST ONE MATERIAL SELECTED FROM THE GROUP CONSISTING OF BERYLLIUM OXIDE AND BERYLLIUM NITRIDE BY SUBJECTING SAID SURFACES TO AT LEAST ONE SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF AIR AND NITROGEN, PLACING IN SAID CRUCIBLE A QUANTITY OF PLUTONIUM, ENCLOSING SAID CRUCIBLE WITHIN A METAL CONTAINER AND AT LEAST PARTIALLY EVACUATING SAID CONTAINER, SEALING SAID CONTAINER, AND THEREAFTER HEATING SAID CONTAINER AND CONTENTS TO A TEMPERATURE SUCH AS TO REACT SAID BERYLLIUM AND PLUTONIUM AND FORM PUBE13. 